The present invention relates to a mass spectrometer and a method of mass spectrometry.
The majority of conventional hybrid quadrupole Time of Flight mass spectrometers comprise a quadrupole mass filter, a fragmentation cell arranged downstream of the quadrupole mass filter and a Time of Flight mass analyser arranged downstream of the fragmentation cell. The mass spectrometer is conventionally used for Data Directed Analysis (DDA) type experiments wherein a candidate parent or precursor ion is identified by interrogation of a Time of Flight (TOF) data set. Parent or precursor ions having a specific mass to charge ratio are then arranged to be selectively transmitted by the quadrupole mass filter whilst other ions are substantially attenuated by the mass filter. The selected parent or precursor ions transmitted by the quadrupole mass filter are transmitted to the fragmentation cell and are caused to fragment into fragment or daughter ions. The fragment or daughter ions are then mass analysed and mass analysis of the fragment or daughter ions yields further structural information about the parent or precursor ions.
The fragmentation of parent or precursor ions is commonly achieved by a process known as Collisional Induced Dissociation (“CID”). Ions are accelerated into the fragmentation cell and are caused to fragment upon colliding energetically with collision gas maintained within the fragmentation cell. Once sufficient fragment ion mass spectral data has been acquired, the mass filter may then be set to select different parent or precursor ions having different mass to charge ratios. The process may then be repeated multiple times. It will be appreciated that this approach can lead to an adverse reduction in the overall experimental duty cycle.
It is known to increase the experimental duty cycle by not performing the step of selecting parent or precursor ions having a specific mass to charge ratio. Instead, the known method repeatedly switches a collision or fragmentation cell back and forth between a fragmentation mode of operation and a non-fragmentation mode of operation without selecting specific parent or precursor ions.
The known approach ideally yields a first data set relating just to precursor or parent ions (in the non-fragmentation mode of operation) and a second data set relating just to fragment ions (in the fragmentation mode of operation). Individual parent or precursor ions observed in the parent ion mass spectrum may then be matched with corresponding fragment ions observed in a fragment ion mass spectrum. The known approach is essentially a parallel process unlike the previously described serial process and can result in a corresponding increase in the overall experimental duty cycle.
A problem associated with the known parallel approach is that the precursor or parent ions which are simultaneously fragmented in the fragmentation mode of operation are not specific and hence a wide range of ions having different mass to charge ratios and charge states will be attempted to be fragmented simultaneously. However, as the optimum fragmentation energy for a given parent or precursor ion is dependent both upon the mass to charge ratio of the ion to be fragmented and also the charge state of the ion, then no single fragmentation energy will exist which is optimum for all the parent or precursor ions which are desired to be fragmented simultaneously. Accordingly, some parent or precursor ions may not be fragmented in an optimal manner or indeed it is possible that some parent or precursor ions may not be fragmented at all.
It might be considered that the fragmentation energy could be progressively ramped or stepped during an acquisition period to ensure that at least some portion of the acquisition time is spent at or close to the optimum fragmentation energy for different parent or precursor ions. However, if this approach were to be adopted then a significant proportion of the acquisition time would still be spent with the parent or precursor ions possessing non-optimum fragmentation energies. As a result, the intensity of fragment ions in a fragment ion mass spectrum is likely to remain relatively low if this approach were adopted. Another consequence of attempting to step or ramp the fragmentation energy during a fragmentation mode of operation may be that some of the parent or precursor ions will remain intact and therefore, disadvantageously, these parent or precursor ions will be observed in what is supposed to be a data set relating entirely to fragment ions.
It is desired to provide an improved mass spectrometer.